1. Field of the Disclosure
The present disclosure generally relates to apparatus and techniques for treatment of spinal disorders, and, in particular, relates to an artificial intervertebral prosthesis which restores both the height and shape of the intervertebral disc space following the removal of a damaged or diseased intervertebral disc while maintaining the natural biomechanics of the spinal motion segment.
2. Description of the Prior Art
The objective in intervertebral disc replacement is to provide a prosthetic disc that combines both stability to support the high loads of the patient's vertebrae and flexibility to provide the patient with sufficient mobility. In attempting to strike this balance, generally, four basic types of artificial intervertebral discs for replacing a part or all of a removed disc have been developed, namely, elastomer discs, ball and socket discs, mechanical spring discs and hybrid discs. Elastomer discs typically include an elastomer cushion which is sandwiched, between lower and upper rigid endplates. The elastomer discs are advantageous in that the elastomer cushion functions similar in mechanical behavior to the removed intervertebral disc tissue. However, a disadvantage of this disc type is that the elastomer cushion experiences long term in-vivo problems stemming from microcracking, which detracts from its usefulness as a replacement option Furthermore, attachment of the elastomer cushion to the endplates presents additional difficulties. Examples of elastomer discs are disclosed in U.S. Pat. Nos. 5,702,450 to Bisserie; 5,676,792 to Ratron; 5,035,716 to Downey; 4,874,389 to Downey; and 4,863,477 to Monson. Ball and socket discs typically incorporate two plate members having cooperating inner ball and socket portions which permit articulating motion of the member! during movement of the spine. The ball and socket arrangement is adept in restoring "motion" of the spine, but, is poor in replicating the natural stiffness of the intervertebral disc. This low stiffness places detrimentally high loads on supporting ligaments and muscles, particularly, in movement involving torsional rotation of the spine. Dislocation and wear are other concerns with this disc type. Examples of ball and socket discs are disclosed in U.S. Pat. No. Nos.: 5,507,816 to Bullivant and 5,258,031 to Salib et al.
Mechanical spring discs usually incorporate one or more coiled springs disposed between metal endplates. The coiled springs preferably define a cumulative spring constant sufficient to maintain the spaced arrangement of the adjacent vertebrae and to allow normal movement of the vertebrae during flexion and extension of the spring in any direction. Disadvantages of the mechanical spring disc types involve attachment of the coiled springs to the metal end plates and associated wear at the attachment points. Furthermore, fibrous Issue growth or encroachment into the coiled springs presents additional difficulties. Examples of mechanical spring discs are disclosed in U.S. Pat. Nos.: 5,458,642 to Beer et al. and 4,309,777 to Patil.
The fourth type of artificial intervertebral disc, namely, the hybrid type incorporates two or more principles of any of the aforedescribed disc types. For example, one known hybrid disc arrangement includes a ball and socket set surrounded by an elastomer ring. This hybrid disc provides several advantages with respect to load carrying ability, but, is generally complex requiring a number of individual components. Furthermore, long term in vivo difficulties with the elastomer cushion remain a concern as well as wear of the ball and socket arrangement.
Another type of intervertebral disc prosthesis is disclosed in U.S. Pat. No. 5,320,644 to Baumgartner. With reference to FIGS. 1-3, the Baumgartner '644 device is a unitary intervertebral disc member 1 made from a strong, elastically deformable material. The disc member 1 has parallel slits 5 each arranged at a right angle to the axis of the disc member. The parallel slits 5 partially overlap one another to define overlapping regions 6 between adjacent slits. The overlapping regions 6 create a leaf spring effect for the transmission of forces from one vertebral attachment surface to the other. In regions of adjacent slits 5 where they do not overlap the spring action on the leaf springs 7 is interrupted by fixation zones 9 of solid prosthesis material. The forces acting on the intervertebral disc are transmitted from one leaf spring plane to the next leaf spring plane via the fixation zones 9.
However, the load paths are inherently abrupt with highly localized transfer of load through the sparsely placed fixation zones 9. There are even instances where the entire load is carried through a single fixation zone 9 in the center of the disc. The abrupt load paths can lead to high stress regions, which can detract from the appropriate biomechanical performance, i.e., strength, flexibility, and range-of-motion, of the prosthesis.
The need exists for a prosthetic disk which is easy to manufacture and provides the proper balance of flexibility and stability through improved load distribution.